Power Amplifier Technology Targets Efficiency

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While network operators look to go green, the pressure is on components suppliers to deliver products that are as energy-efficient as possible.

Energy efficiency is becoming increasingly important as a means of reducing greenhouse emissions, especially as data traffic has grown due to the unprecedented development and growth in global wireless communications systems over the last decade. Network operators look for the highest possible efficiency in the networks they deploy, which puts pressure on components suppliers to deliver products that are as efficient as possible, especially higher-power active components such as power amplifiers (PAs).

A significant portion of the wireless base station power budget is consumed by the transmit RF PA. Since the PA can account for more than one-half of the base station's power usage, PAs have been the focus of efforts to improve power-added efficiency (PAE). The PAE is a measure of how well a PA can convert DC power into RF/microwave power. A PA’s PAE is highly dependent upon the amplifier’s gain, with high gain resulting in higher levels of PAE.

Gallium nitride (GaN) semiconductor technology has proven to be extremely well suited for RF/microwave PAs. GaN can achieve transistors with power densities that are as much as five times higher than conventional gallium-arsenide (GaAs) devices, resulting in improved gain and efficiency. As the manufacturing costs of GaN decrease, the technology is being chosen for more high-power and high-frequency applications due to its superior performance characteristics. A number of companies are utilizing the benefits that GaN has to offer to produce highly efficient PAs well suited for cellular communications base stations and other wireless applications.

Cree, Inc. and ETA Devices have partnered to produce the world's most efficient reported PA. By taking advantage of the performance enhancements that Cree’s GaN high-electron-mobility-transistor (HEMT) RF devices offer, the next-generation of ETA Devices’ PAs are capable of providing efficiencies that exceed 70 percent under a 4G LTE modulation format. In comparison, silicon laterally diffused metal-oxide-semiconductor (LDMOS) transistors that are employed in current-generation mobile base-station PAs can only provide efficiencies as high as 45%. This new PA solution has the potential to save mobile operators 60 terawatt-hours (TWh) of energy per year. In addition, it could also save as much as 50% of the $36.5 billion used to power mobile base stations each year.

NXP Semiconductors has also responded to the demand by wireless network operators for more efficient and environmentally friendly solutions. NXP is engaged in efforts to develop switch-mode PA (SMPA) technology. The power transistors in an SMPA are driven into saturation, which cause the transistors to operate as nonlinear switching devices and achieve higher efficiency than a linear-mode amplifier. NXP has developed an amplifier topology based on the Chireix structure, which employs two Class-E amplifiers in parallel (Fig. 1). This structure, invented by Henri Chireix in 1935, utilizes vector addition of two signals of equal amplitude and different phases. The output is optimally combined by means of an out-of-phase combiner. The development of GaN transistors has enabled the Class-E Chireix topology to achieve high power efficiency. NXP believes that SMPAs will deliver significant increases in efficiency in future mobile communications infrastructure applications.

Efficiency is also an important factor in the deployment of high-power broadcast transmitters, as energy consumption can account for more than one-half of all the costs that are associated with the operation of these transmitters. Rohde & Schwarz recently introduced its model THU9, which is a liquid-cooled high-power transmitter that establishes new performance benchmarks for high-power broadcast transmitters. The THU9 integrates Doherty amplifier technology, which allows users to save up to 50% of energy costs compared with conventional transmitters.